This application is a US National filin of PCT Application PCT/IL98/00075, filed Feb. 12, 1998.
The present invention relates generally to computerized tomographic (CT) imaging, and specifically to multi-slice CT scanners having helical scan paths.
Helical-path CT scanners are well known in the art. Generally, such scanners comprise an X-ray tube, mounted on an annular gantry, so as to revolve continuously about a subject being imaged. The subject lies on a bed, which is translated continuously through the gantry simultaneously with the tube""s revolution. An array of X-ray detectors on the opposite side of the subject from the X-ray tube receive radiation transmitted through the subject. The detectors generate signals proportional to the attenuated X-ray flux incident thereon. The signals are pre-processed to produce attenuation data, which are used in reconstructing orimages of the subject. In xe2x80x9cthird-generationxe2x80x9d scanners, the array of detectors is mounted on the gantry so as to revolve along with the X-ray tube, whereas in xe2x80x9cfourth-generationxe2x80x9d scanners, the detectors are arrayed in a ring, which is generally stationary.
The axis of translation of the bed (conventionally the Z-axis) is generally parallel to the long axis of the subject""s body, which is typically perpendicular to the plane of revolution of the tube. Thus, the path of the X-ray tube relative to the subject generally describes a helix about this axis, and X-ray attenuation data received from the X-ray detector array similarly correspond to a series of helically-disposed angular xe2x80x9cviewsxe2x80x9d through the subject.
In order to reconstruct a planar cross-sectional image slice of the subject at a desired axial position, based on the helical-scan views, effective attenuation values for each of a plurality of points around a circumference of such a planar slice are derived by interpolation between data received in the original helical-path views. For each of the plurality of points, the respective effective attenuation values thus correspond to the approximate attenuation along rays within the planar slice that pass through the point. For 360xc2x0 reconstruction, as is known in the art, the plurality of points are distributed around the entire circumference of the slice. For 180xc2x0 reconstruction, also known in the art, the points are distributed over an extent equal to half the circumference plus the xe2x80x9cfan angle,xe2x80x9d i.e., the angular field of view covered by the fan-shaped X-ray beam. (For convenience in the following discussion, we will refer to the total angular extent of all the views that are collectively used in the reconstruction of a complete planar slice as the xe2x80x9creconstruction angle,xe2x80x9d typically 360xc2x0 or 180xc2x0.) The interpolated effective attenuation values are filtered and back-projected to produce the cross-sectional image.
In general, to derive effective attenuation values for a given view angle and axial position, it is necessary to interpolate between actual data from at least two different views at that view angle, bracketing the axial position of the planar slice. The two views are acquired in successive scan segments, separated by the fall reconstruction angle. Because data from two different scan segments must typically be combined to reconstruct each planar image slice, data from the earlier of the two scans must be stored in buffer memory. Two full scans, comprising twice the total reconstruction angle, are thus needed to produce a single cross-sectional image. Reconstruction of each planar image slice lags behind acquisition of the data in the earlier of the two scans by at least the time it takes the tube to make a full scan around the subject. By comparison, in axial (non-helical) CT scanners, successive views may be processed in a xe2x80x9cpipelinexe2x80x9d data flow, so that the slice image is displayed with a minimal delay after completion of a single scan. The lag in helical scan reconstruction is particularly disadvantageous when CT imaging is used to track the progress of a physiological process, such as the flow of a contrast material.
Multi-slice helical-path scanners are known in the art. For example, U.S. Pat. No. 5,485,493, which is incorporated herein by reference, describes a multiple-detector-ring spiral scanner with relatively adjustable helical paths, in which two adjacent, parallel slices are acquired along two parallel paths simultaneously or sequentially. Data corresponding to planar slices are derived by interpolating between data acquired along the two helical paths.
U.S. Pat. No. 5,524,130, the disclosure of which is incorporated herein by reference describes a number of methods for utilizing a single detector ring scanner to provide successive axially spaced slices with reduced time between reconstruction of the slices. Some of these methods appear to utilize partial scan data from one scan to replace data from a second scan taken at a different time for reducing the reconstruction time.
It is an object of the present invention to provide a method for on-line image reconstruction of helical-scan CT data.
In one aspect of the present invention, the CT data comprise multiple-slice CT data, acquired using a multi-row detector array.
In preferred embodiments of the present invention, a multi-slice helical-scan CT scanner comprises an X-ray tube, mounted to revolve on an annular gantry about a bed on which a subject lies, and a detector array. The bed is advanced through the gantry along a translation axis that is generally parallel to the long axis of the subject""s body. The X-ray tube thus describes a generally helical trajectory about the subject""s body and irradiates the subject from multiple positions, or xe2x80x9cviews,xe2x80x9d along this trajectory. The detector array preferably comprises two or more parallel rows of X-ray detector elements, each row having a long axis disposed in a generally circumferential direction with respect to the long axis of the subject""s body. The detector elements receive radiation that has passed through the subject""s body at each of the views and generate signals responsive to attenuation of the X-rays.
Preferably, for 360xc2x0 CT image reconstruction, the width of the detector array which is exposed to radiation, measured in a direction parallel to the translation axis, is substantially greater than the pitch of the helical trajectory. For 180xc2x0 reconstruction, the width of the detector array which is exposed to radiation is substantially greater than half the pitch of the helical trajectory.
For each view, the two or more rows of the array simultaneously generate corresponding line attenuation signals. These signals are preprocessed, as is known in the art. The resultant line attenuation data are interpolated to generate geometrically-corrected effective attenuation values, which are associated with planar slices through the body that are generally perpendicular to the translation axis. The corrected effective attenuation values are filtered and back-projected to calculated CT values, which are used to update cross-sectional CT images substantially in real time.
In preferred embodiments of the present invention, an effective attenuation value is calculated for each of a plurality of points on a periphery of each planar slice by weighted interpolation between first and second measured attenuation values, taken from respective first and second line attenuation signals within a single, multiple-slice view. The values are preferably calculated by linear interpolation, but may alternatively be calculated using non-linear or other helix interpolation schemes known in the art. The first and second line attenuation signals are derived respectively from data received simultaneously from first and second rows of detector elements. Thus, there is no need for any additional buffer memory to store data from a preceding scan (although the data may be stored if desired), and a complete cross-sectional CT image may be acquired and immediately reconstructed within a time window corresponding to a single rotation of the tube over the reconstruction angle.
In some preferred embodiments of the present invention, a time sequence of images of a single planar slice at a selected axial position is reconstructed, as described above, using data acquired from a helical scan in a vicinity of the position. Each image in the sequence is reconstructed from data acquired during a different time xe2x80x9cwindow.xe2x80x9d The successive images in the sequence are preferably processed and displayed on line, as described, for example, in Israel patent application number 120227 entitled xe2x80x9cReal-time Dynamic Image Reconstruction,xe2x80x9d filed on Feb. 20, 1997 and a corresponding PCT patent application of the same title filed on the same day as the present application. The disclosure of both these applications incorporated herein by reference.
In some preferred embodiments of the present invention, a series of planar slices are reconstructed at a corresponding series of axial positions covering a range of interest within the subject""s body. Data are acquired from a helical scan in a vicinity of a first position in the series. A first planar slice at the first position is then reconstructed and displayed, as described above, while data are acquired in a vicinity of a second, subsequent position in the series. This process of acquisition, reconstruction and display is preferably repeated with respect to a third position, fourth position, and so forth, over the entire series.
In these preferred embodiments, the data acquired in the vicinity of each of the positions is preferably stored and used in subsequent image processing, reconstruction and display. Thus, for example, at least some of the data acquired in the vicinity of the first slice may be incorporated in the reconstruction of the second slice, and so forth. By using such overlapping data sets in reconstructing image slices at successive positions, closely-spaced slices may be produced, so that features within the subject""s body may be seen in greater detail. Additionally, the CT scanner may be adapted to reconstruct two or more such slices simultaneously, so that multiple slices may be reconstructed and displayed in rapid succession.
It will be appreciated that the principles of the present invention are equally applicable to third- and fourth-generation CT scanners and to various image reconstruction methods, including 180xc2x0, 360xc2x0, fan beam and parallel beam reconstruction, as are known in the art. Moreover, although in the preferred embodiments described herein, the Z-axis, along which the bed advances, is generally perpendicular to the plane of revolution of the tube, the principles of the present invention may similarly be applied to CT image reconstruction using angled helical scan paths, as described in a PCT patent application PCT/IL97/00069, filed on Feb. 20, 1997, entitled xe2x80x9cHelical Scanner with Variably Oriented Scan Axis,xe2x80x9d which is assigned to the assignee of the present invention, and whose disclosure is incorporated herein by reference. This application designates the US.
It will be appreciated that, 180 degree and 360 degree rotations are utilized in the above description for simplicity. However, actual data collection may extend to the reconstruction angle plus the fan beam angle under certain reconstruction schemes. Such a reconstruction angle is included within the meaning of the term xe2x80x9cgenerally equal to the reconstruction anglexe2x80x9d as that term is used herein.
There is thus provided, in accordance with a preferred embodiment of the invention, a method for reconstructing a planar image slice in a CT scanner having a predetermined reconstruction angle and including a detector array having n rows, n being an integer greater than 1, said method comprising:
acquiring X-ray attenuation data from the detector array along a predetermined portion of a helical scan path in a vicinity of an axial position corresponding to the planar image slice, wherein the predetermined portion has an angular extent that is generally equal to the reconstruction angle; and
processing the data to reconstruct an image of the slice, using data acquired substantially only along the predetermined portion of the scan path.
Preferably, acquiring X-ray data from the detector array comprises acquiring data from detector elements in at least two rows of the detector array, and processing the data comprises interpolating the data acquired from the at least two rows.
Preferably, acquiring X-ray attenuation data along the predetermined portion of the helical scan path comprises acquiring data along a helical path having a pitch that is less than or equal to (nxe2x88x921)/n times a width of the detector array, measured in the axial direction, and wherein processing the data to reconstruct the image comprises processing the data using 360xc2x0 reconstruction.
Alternatively, acquiring X-ray attenuation data along the predetermined portion of the helical scan path comprises acquiring data along a helical path having a pitch that is less than or equal to 2(nxe2x88x921)/n times a width of the detector array, measured in the axial direction, and wherein processing the data to reconstruct the image comprises processing the data using 180xc2x0 reconstruction.
Preferably, processing the data to reconstruct the image comprises processing the data for a given portion while the data is being acquired.
A preferred embodiment of the method comprises acquiring data along a second, successive portion of the scan path, wherein processing the data to reconstruct the image comprises processing the data acquired along the predetermined portion of the helical scan path while acquiring the data along the second portion. Preferably, processing the data acquired along the predetermined portion of the helical scan path comprises displaying the image while acquiring data along the second portion. Preferably, the method includes processing the data acquired along the second portion of the scan path to reconstruct a second image. Preferably, the method further includes storing at least some of the data acquired along the predetermined portion of the scan path, wherein processing the data acquired along the second portion of the scan path comprises processing the data acquired along the second portion together with the stored data acquired along the predetermined portion to reconstruct the second image.
In a preferred embodiment of the invention, processing the data acquired along the second portion of the scan path to reconstruct the second image comprises deriving CT values from the data and averaging the values with other CT values derived from the attenuation data acquired along the predetermined portion of the scan path.
In a preferred embodiment of the invention, processing the data acquired along the second portion of the scan path to reconstruct the second image comprises deriving CT values from the data and subtracting the values from other CT values derived from the attenuation data acquired along the predetermined portion of the scan path.
Preferably, processing the data acquired along the second portion of the scan path to reconstruct the second image comprises producing an image of the slice having improved image quality. Preferably, processing the data acquired along the second portion of the scan path to reconstruct the second image comprises producing an image showing a change in the body of a subject in a vicinity of the slice.
In a preferred embodiment of the invention, processing the data comprises displaying the image and the second image in a cine mode.
There is furter provided, in accordance with a preferred embodiment of the invention, a method for reconstructing a planar image slice in a helical mode CT scanner having a predetermined reconstruction angle and including a detector array having a plurality of rows detectors, the method comprising:
reconstructing said slice using data acquired during a first time window to form a first image; and
reconstructing said slice using data acquired during a later time window to form a second image.
Preferably, the method includes successively displaying said first and second images.
In a preferred embodiment of the invention, wherein the first and second images are reconstructed using data acquired by more than one row of detectors and wherein at least one of the rows used to reconstruct one of the images is different from any of the rows used to reconstruct the other of the images.
In a preferred embodiment of the invention, the method includes reconstructing and displaying a third image of the slice from data acquired during a third time window.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: